Replace 'signature' argument in apply_ufunc with *_core_dims

[shoyer]

This results in a much easier to use function signature than the previous
signature argument which required triply nested lists.

e.g., new:

def inner_product(a, b, dim):
    return apply_ufunc(_inner, a, b, input_core_dims=[[dim], [dim]])

vs. old:

def inner_product(a, b, dim):
    return apply_ufunc(_inner, a, b, signature=[[[dim], [dim]], []])

I also was able to remove the code related to parsing gufunc signatures from strings.

• tests added / passed
• passes git diff upstream/master | flake8 --diff

[rabernat]

I like the new syntax much better. I spent 45 minutes trying to figure out the correct signature.